The HARQ protocol is a variation of the ARQ error-control protocol. In standard ARQ, Error Detection (ED) information bits are added to data to be transmitted, such as Cyclic Redundancy Check (CRC) bits. In HARQ, Forward Error Correction (FEC) bits are further added to the ED bits. HARQ is used in High-Speed Downlink Packet Access (HSDPA), High-Speed Uplink Packet Access (HSUPA), and 3GPP Long Term Evolution (LTE) data transmissions.
To enhance radio link reliability, the HARQ protocol requires the receiver of a data transmission to communicate an ACK or NACK indicator back to the sender. When an ACK indicator is sent back, the sender can proceed to transmit the next data packets. When a NACK indicator is received by the sender, coded bits corresponding to the previous transmission are transmitted to the receiver.
Standard document 3GPP TS 36.212 V9.1.0 (2010-03), 3rd Generation Partnership Project, Technical Specification Group Radio Access Network, Evolved Universal Terrestrial Radio Access (E-UTRA), Multiplexing and channel coding (Release 9) specifies coding, multiplexing and mapping to physical channels for E-UTRA. Therein, coding of HARQ feedback information is described.
In the LTE system, a User Equipment (UE) is notified by the network about PDCCH DL data transmissions. The PDCCH is a transmission channel that is used to transfer control information to UEs. The PDCCH defines how the paging channel and shared DL channels are configured and it defines uplink (UL) transmission scheduling information to help coordinate access control to the radio system. Upon receipt of a PDCCH in a particular subframe T, the UE is required to send back the ACK/NACK indicator in a Physical Uplink Control Channel (PUCCH) at a subsequent subframe T+k. PUCCH is generally used to transport user signaling data from one or more UEs that can transmit on the control channel. PUCCH transports the acknowledgment responses and retransmission requests, sends service scheduling requests, and transfers channel quality information measured by the UE to the system. In the Frequency Division Duplex (FDD) mode of LTE, a fixed time delay k can be used since the UL radio resources are always available. Furthermore, each ACK/NACK feedback indicator in the UL direction corresponds to a unique DL transmission.
In 3GPP LTE Rel-10 (“LTE-Advanced”), bandwidths larger than 20 MHz are supported while backwards compatibility with 3GPP LTE Rel-8 is supported. This is achieved by aggregating multiple component carriers, each of which can be 3GPP LTE Rel-8 compatible, to form a larger overall bandwidth for an UE that is 3GPP LTE Rel-10 compatible. FIG. 1 shows an aggregated carrier bandwidth of 100 MHz, wherein each of the component carriers is separately processed.
FIG. 2 schematically illustrates the Radio Link Control (RLC), Medium Access Control (MAC), and Physical (PHY) layers for an LTE system having multiple component carriers. As can be seen from FIG. 2, HARQ is operated separately on each component carrier. For HARQ operation, acknowledgements informing the transmitter on whether the reception of a transport block was successful or not are provided. For this, multiple acknowledgement messages, i.e., one per component carrier, are transmitted. In case of spatial multiplexing, an acknowledgement message corresponds to two bits as there are two transport blocks on one component carrier. In absence of spatial multiplexing, an acknowledgement message is a single bit as there is only a single transport block per component carrier.
The number of aggregated component carriers as well as the bandwidth of the individual component carrier may be different for UL and DL transmissions. A symmetric configuration refers to the case where the number of component carriers in DL and UL is the same whereas an asymmetric configuration refers to the case that the number of component carriers is different. The number of component carriers configured in a cell may be different from the number of component carriers seen by an UE. For example, an UE may support more DL component carriers than UL component carriers, even though the cell is configured with the same number of UL and DL component carriers. To reduce UE power consumption and implementation costs, PUCCH is transmitted on a semi-statically configured UL component carrier (a so-called “anchor carrier”).
In 3GPP systems, fast activation and de-activation of DL component carriers is supported. Thereby, the UE is enabled to monitor only those component carriers that the network schedules most of the time for the UE. Activation and de-activation can be based on L1/L2 control signaling or MAC control elements. Upon activation of a DL component carrier, an UE has to be able to receive PDSCH thereon. The PDSCH is used to send common user data and control information (such as paging messages) to all UEs operating within its coverage area. Therefore, the maximum number of simultaneous DL assignments that an UE can receive is limited by the number of activated component carriers (denoted herein by “n”). The actual number of assignments can vary between 0 and the number of activated component carriers. Furthermore, even if an UE is scheduled on n component carriers, the UE may only be able to successfully decode less than n PDCCHs carrying DL assignments. This event is in the following referred to as “DTX”.
With n activated component carriers and all component carriers configured for single-codeword transmissions, it is necessary to signal three possible HARQ messages per component carrier, i.e., {ACK, NACK, DTX}. Excluding the case where none of the PDCCHs is received correctly by the UE, there are 3n−1 possible HARQ messages, which require ┌ log2(3n−1)┐ binary bits for representation. An example of HARQ message encoding for n=3 component carriers with single-codeword scheduling is shown in Table 1. In Table 1, A=ACK, N=NACK, and D=DTX.
TABLE 1HARQ message encoding for n = 3 component carriers withsingle-codeword schedulingIndicatorBinaryAAA00000NAA00001DAA00010ANA00011NNA00100DNA00101ADA00110NDA00111DDA01000AAN01001NAN01010DAN01011ANN01100NNN01101DNN01110ADN01111NDN10000DDN10001AAD10010NAD10011DAD10100AND10101NND10110DND10111ADD11000NDD11001
Table 1 shows that the translation from a HARQ message to its binary representation is rather irregular. The transformation between the HARQ messages and the binaries is provided by means of a look-up table. For the maximum number of five activated component carriers (as envisioned for 3GPP LTE Rel-10), the look-up table has 242 entries.
The numbers of possible HARQ messages grows significantly when the system is operated with multiple antennas. For a simplified example, it is assumed that all n activated component carriers use dual-codeword transmissions. Thus, five possible HARQ messages have to be signaled per component carrier, i.e., {(ACK, ACK), (ACK, NACK), (NACK, ACK), (NACK, NACK), (DTX)}. Hence, there are 5n−1 possible HARQ messages, which require ┌ log2(5n−1)┐ binary bits for representation. Thus, for five activated component carriers (as envisioned for 3GPP LTE Rel-10), a general look-up table can have 55−1=3124 entries.
Since different component carriers can exhibit different operating conditions due to different transmission and interference characteristics, it is necessary to allow flexibility in assigning different numbers of transmitted codewords to different component carriers for higher system operation performance. Standard document 3GPP TS 25.212 V9.2.0 (2010-03), 3GPP, Technical Specification Group Radio Access Network, Multiplexing and Channel Coding (FDD), (Release 9) discloses to signal seven possible HARQ messages per component carrier, i.e., {(ACK, ACK), (ACK, NACK), (NACK, ACK), (NACK, NACK), (ACK), (NACK), (DTX)}.
Table 2 shows a HARQ message encoding table for n=2 component carriers for High Speed Packet Access (HSPA) Release 9. Table 2 has been reproduced from Table 15C.2 of document 3GPP TS 25.212 V9.2.0. Table 2 contains 72−1=48 entries. For the maximum number of five activated component carriers (as envisioned for 3GPP LTE Rel-10), a general look-up table can have 75−1=16806 entries.
TABLE 2HARQ message encoding for n = 2 component carriers for HSPA (Release 9)IndicatorBinaryA/D1111111111N/D0000000000AA/D1010111101AN/D1101010111NA/D0111101011NN/D1001001000D/A0000001111D/N1111110000D/AA1000100011D/AN0100001101D/NA0001111110D/NN1111100100A/A1101000011A/N0011101001N/A1001011100N/N0110010101A/AA1010011000A/AN1001010101A/NA0011101001A/NN0111010011N/AA1101001010N/AN1100010110N/NA0110101010N/NN0010110101AA/A0110000100AA/N1110011010AN/A1011100110AN/N0011010001NA/A0101111100NA/N1100100001NN/A0000110010NN/N0100011001AA/AA0110110111AA/AN1011001111AA/NA1101111001AA/NN0111011100AN/AA0001100101AN/AN1110000001AN/NA1000010100AN/NN0011010001NA/AA1100101110NA/AN0010101000NA/NA1011110010NA/NN1110011010NN/AA0101000010NN/AN0010000110NN/NA0100110000NN/NN0000011011
Existing encoding schemes require a fixed-length error correction coding scheme for a particular number of activated component carriers. For example, a forward error correction code for n=5 activated component carriers has to be able to carry 12 HARQ information bits regardless of how many PDCCHs are transmitted from the network and whether Multiple Input Multiple Output (MIMO) transmissions are scheduled for any of the component carriers. Thus, even in case only one codeword transmissions are scheduled on two of the five available component carriers, 12 HARQ information bits are needed. Therefore, known fixed-length encoding schemes are limited with regard to performance improvements.
Moreover, existing encoding schemes with fixed-length error correction coding do not provide sufficient error protection levels to the ACK/NACK and DTX indicators. For example, looking at the HARQ message encoding for n=3 component carriers with single-codeword scheduling illustrated in Table 1, serious HARQ operation errors can result if the error correction decoder provides only a single bit error. If e.g. “01000” is mistaken for “01001”, the network assumes that the data blocks for the first two component carriers are received correctly even though the UE did not receive the corresponding PDCCH at all. By means of such errors, significant data throughput losses may occur.
Thus, the problem of enhanced error protection for encoding DL HARQ feedback information arises.